Although it is possible to change gratings without the ICL interface running
this procedure is rather involved, and should only be performed with the advice
of technical staff. With the ICL interface running the procedure is fairly
straightforward.

The grating angle or angles must first be moved to the change position, in this
position the grating cell is parallel to the back of the spectrograph, and to
the access doors, and the gratings can be removed from their cells and lifted
straight through the access doors. An interlock prevents the grating cells
being unclamped with the gratings at any other angle.

The procedure for changing gratings is invoked by typing:--

CHANGE

at the ICL interface. A list of items which can be changed with
this procedure is given, the relevant grating should be selected.
The user will then be offered access to the
spectrograph, if the response to this question is ``YES'' then the selected
grating will be moved to the change position (grating angle 35000) and
both grating doors will be unlocked.

It is now possible to open the grating doors, which are located on the face
of the spectrograph opposite the long arm. The door for the blue grating is
in the upper left hand corner of this face; and the door for the red grating
is in the lower right hand corner. When the door is opened the grating should
lie approximately parallel to this spectrograph face, with the handles towards
the door.

The grating clamps are operated by nitrogen pressure, and are
activated by a toggle switch. For the red grating the switch is
situated above the grating cell, and for the blue grating below the grating
cell. If this toggle switch is moved to the down position the grating
clamps will release, this
should only be done with the telescope at the zenith, and the user should keep
one hand on one of the handles on the grating itself
while releasing the clamps. The grating can then
be withdrawn from the cell, and taken out of the spectrograph. All except the
tallest of observers should use a set of steps while changing the blue grating,
its cell is both higher off the ground and deeper inside the spectrograph than
that of the red grating.

Gratings are normally kept in wooden boxes in a metal storage cabinet on the
observing floor. The boxes are clearly labelled with the identifier of the
grating they contain. The gratings should be kept in these boxes whenever
they are not in use, and under no circumstances should they be left out on
the observing floor. Users should never touch the surface of a grating.
Any blemishes on the surface should be reported to the telescope manager, users
should never attempt to clean them off.

Replacing a grating in the cell is the reverse of removing one. ISIS gratings
are always used blaze-to-collimator, so the arrow showing the direction of
the blaze angle should always point towards the centre of the spectrograph.
The writing on the grating should be the right way up if it is in the red cell,
and upside down if it is in the blue cell. The grating should be placed in the
cell, ensuring that it is properly seated. The toggle switch should then be
moved to the up position; the clamps will come on. If the clamps release immediately after coming on, then the grating
is not properly seated.

After the change is completed the user should press the return key at the
ICL interface. The grating doors will then be locked, provided that they
are closed correctly. The user should then type in the name of the grating
loaded in response to the question. The ISIS database will then be updated,
and the name of the grating loaded should appear on the mimic. It is
important that the correct grating identification is loaded in the database,
otherwise the procedure CENWAVE will not work correctly.

Users should never rely on the information contained in the database,
nor that on the whiteboard in the control room, being correct when they first
take over the system. The database could be incorrect if a different version
of the software has been in use, as different versions of the software have
separate databases. The only way to check that the correct grating is loaded
is to run the change procedure, open the grating door, and look.